In potable water testing and food safety testing, the presence or absence of coliform bacteria is considered to be important evidence of quality, and the amount of coliform bacteria permitted in drinking water and in certain foods (for example, dairy products) is regulated in many countries around the world. Coliform bacteria include fecal coliforms, such as Escherichia coli. The presence of fecal coliforms in a food or water sample is used as a primary indicator of fecal contamination of the food or water and of the possible presence of other pathogenic microorganisms.
Methods for detecting, identifying, and/or enumerating microorganisms in water samples can be found, for example, in the compendium Standard Methods for the Examination of Water and Wastewater (“SMEWW”), 21st Edition, which is a joint publication of the American Public Health Association, the American Water Works Association, and the Water Environment Federation. SMEWW describes a membrane filtration technique that is commonly used in water testing to obtain a direct count of the microorganisms present in relatively low concentration in relatively large volumes of water. In carrying out this technique, a certain volume of water is passed through a membrane filter, the membrane is incubated in a culture medium or device for a certain period of time, and the resulting microorganism colonies are then counted. Membrane filtration techniques are useful in monitoring the microbiological quality of samples from processes intended to produce drinking water, as well as samples from a variety of natural, unprocessed water sources.
Methods for detecting, identifying, and/or enumerating microorganisms in food samples often vary according to the nature of the food and the types of microorganisms that are likely to be found in the samples. Compendia of methods for testing food samples include Standard Methods for the Examination of Dairy Products, 27th Edition, published by The American Public Health Association, Washington, D.C., and the Bacteriological Analytical Manual (“BAM”), published by the U.S. Food and Drug Administration, Washington, D.C. Solid foods are usually suspended in aqueous media and mixed and/or pulverized to obtain a liquid homogenate of the food material, which can be used in methods of quantitative microbial analysis.
The above-referenced methods, however, are generally relatively expensive, involve multiple steps, and/or require relatively sophisticated instrumentation and/or relatively highly trained personnel. For example, most membrane filtration techniques require sterilized apparatus, vacuum manifolds, and manual interpretation of results. Another drawback is that membrane filters can become blocked by small particulates (for example, silt, dust, rust, or other suspended particulates).
Techniques such as centrifugation require specialized power equipment for sampling large volumes (for example, volumes greater than 50 milliliters) and require relatively extended periods of time to recover relatively low numbers of microorganisms from such volumes. Culture devices used in microbiological analysis of samples often can accommodate only relatively small sample inoculum volumes (for example, about 1 milliliter). Such limitations can be particularly problematic in the area of water testing, as U.S. Environmental Protection Agency water quality testing regulations, for example, stipulate the testing of large (100 milliliter) water sample volumes.